1. Field of the Invention
This invention relates to a method of heating for use in the heat treatment of polyvinyl chloride and the like, more particularly to a method of high-speed heating employing microwave energy to generate heat with high uniformity in the interior of the material being treated.
2. Description of the Prior Art
In the production of polyvinyl chloride (PVC) for processing into film, sheet, floor covering material, wallpaper etc., there is a recent trend toward the use of larger and higher speed production equipment for the purpose of increasing production volume and decreasing production cost.
In the manufacture of PVC products, heat treatment processes are inevitably required for such purposes as plasticizing, gellating, melting and foaming the PVC. Conventionally, PVC has been heated using such devices as rollers, Banbury mixers, extruders, etc. which generate heat mechanically (by shearing, friction or thermal conduction) or, in the case of gellation and foaming, using an external heating system employing direct firing or some other heat source such as electricity, infrared radiation or steam.
These conventional heating methods have, however, often proved to be less than satisfactory from the points of high-speed production, energy conservation (economy), heating uniformity, etc. Moreover, a need has been felt for a heating method which is capable of high-speed temperature control (high-speed response) and thus of providing high efficiency.
Generally speaking, the hardness (flowability) of PVC can be freely controlled by adjusting the amount of plasticizer added thereto: PVC containing plasticizer at a ratio relative to the total of PVC and plasticizer of 0%, 10-30% and 30% or more being called hard, semihard and soft PVC respectively. Moreover, as the temperature of PVC rises, it becomes soft, with an attendant decline in tensile strength, Young's modulus and hardness, and an increase in elongation and elasticity. With further rise in temperature, it increases in flowability and passes into a molten state, whereafter, if no stabilizer is present, it will begin to undergo pyrolysis. The temperature at which softening and melting commences varies greatly with the quantity of stabilizer addition and the PVC composition.
FIG. 1 shows a block diagram of a conventional processes for manufacturer of PVC film or sheet. Briefly stated, these film or sheet manufacturing processes consist of a blending step b, a mixing and kneading step c, a warming step d, a conveying step e, a rolling step f, a cooling step g and a winding step i.
The PVC raw materials a, namely the plasticizer, coloring agent, filler, lubricant, etc., are accurately metered and forwarded to the blending step b where they are uniformly dispersed, mixed and blended in a blender. The blended mixture is then forwarded to the mixing and kneading step c where it is kneaded by kneading rolls or the like and passed on to the warming step d where it is warmed by warming rolls so as to be thoroughly plasticized. The plasticized material is then slit into ribbon-like strips which are loaded on an oscillating conveyor in the conveying step e for transport to the rolling step f (sometimes being passed through a metal detector for the removal of foreign material on the way). They are then fed into the top bank of calender rolls.
Although the type and number of calender rolls used depends on the type and blending ratio of the raw materials, it is most common to use four or five L-type, inverted L-type or Z-type calender rolls. In calendering, the roll temperature is held between 160.degree.-180.degree. C. depending on the composition, with the temperature of the fourth roll generally being set 5.degree.-20.degree. C. higher than that of the second pair of rolls. The temperature of each roll is accurately controlled by forced circulation of high-pressure steam of high-pressure hot water.
The PVC material is rolled into film or sheet form by these rollers, passed on to the cooling step g for cooling and setting, trimmed along both edges to a constant width at the trimming step h and then cut after being wound to a prescribed length on a winder in the winding and cutting step i. The scraps from the trimming step i are sent to the pulverizer of a recovery step j for pulverization and recycling to the blending step where the pulverized material is mixed with the raw materials.
For obtaining a high-quality and stable film or sheet according to this manufacturing process it is essential to constantly maintain the top bank of the calender rolls under optimum rotating condition and this depends on constantly maintaining the temperature and material supply rate at optimum levels. These are extremely important factors in determining the quality of the final product.
The PVC is thus processed to the proper degree of plasticity, flowability and fusion by kneading, warming, extrusion, rolling and the other steps constituting the manufacturing process, and in the case of soft PVC containing a large amount of plasticizer it is relatively easy to obtain the desired plasticity, gellation and fusion with the equipment used for this purpose (kneading rolls, extruder, calender rolls, etc.). In the case of hard PVC containing no or only very little plasticizer, on the other hand, since the processing temperature and viscosity are higher and the required amount of driving power is greater, it is necessary to use higher strength processing equipment. As a result, production cost is increased by the need for more energy and more expensive equipment, while problems also arise from the point of processing technology since higher levels of technical expertise and operating skill are required to cope with the narrower range of optimum operating conditions dictated by the fact that hard PVC is more susceptible to pyrolysis than soft PVC and with the consequent need to add greater amounts of stabilizer. Because of this, the idea of using microwaves to heat PVC by high-speed vibration of the PVC molecules was taken up and tested.
The term "microwaves" refers generally to extremely short-wavelength electromagnetic waves, more specifically to such waves with wavelengths between 0.3-30cm. These waves are very readily absorbed by dielectrics having a polar radical such as water, alcohol etc. and when such a dielectric is irradiated by microwaves its molecules are subjected to high-speed internal vibration which results in the generation of heat. Microwaves thus provide a highly efficient heating effect and are therefore widely utilized in microwave ovens and other devices.
Conventionally when microwave heating has been used for PVC, it has been applied to the wound film or sheet in the winding step. When microwave energy is used in this way to bring the PVC from room temperature up to the gellation, fusion or foaming temperature, heat generation concentrates in certain local regions, inevitably causing these regions to rise to an abnormally high temperature and undergo pyrolysis and, eventually, even carbonization. Thus, up to now there has been no method for carrying out uniform heating using microwaves.